Dihydrofolate reductase is the site of action of several important drugs including trimethoprin, pyrimethamine and the widely used cancer chemotherapeutic agent methotrexate. We attempt to understand the mechanism of action as well as inhibition of this enzyme and also to elucidate its molecular genetics. Despite small amounts of the enzyme in normal animal tissues and the resulting problems associated with purification and isolation of a homogeneous crystalline enzyme under these conditions, we can now report the 2.9 Angstroms structure of the chicken liver dihydrofolate reductase in a ternary complex with NADPH and a phenyltriazine inhibitor - the first structural determination of dihydrofolate reductase from a eukaryotic species. The same purification procedure, affinity chromatography and isoelectric focusing, is currently being used in an attempt to isolate the reductase from certain human tissues, however these studies are complicated by even smaller levels of enzyme activity than previously observed in avian and bovine liver.